Drew Anthony Smith/Getty Images(KILLEEN, Texas) -- The general in charge of Fort Hood said Thursday there was an "awful lot of heroism" during the shooting rampage by Spc. Ivan Lopez, and he cited a female military police officer and a chaplain for their actions.

"There were several instances here of clear heroism," Gen. Mark Milley told a news conference at the entrance to the fort where Lopez killed three soldiers and wounded 16 before killing himself. "I would cite that female MP...There were others, folks inside this building who saved lives."

The general mentioned, "a chaplain who shielded others... He broke some windows to get people to safety," Milley said.

He declined to identify the MP or the chaplain, but said the military police officer was within 20 feet of Lopez when he put up his hands, and then reached for his gun.

"She engaged him with small arms fire at which time the shooter fired a self-inflicted gunshot wound," he said. "She did fire her weapon," Milley said at another point.

Milley also praised two wounded soldiers who, "demonstrated heroism and had the presence of mind" to make the first 911 call at 4:16 p.m. on Wednesday.

The shooter's motive is under investigation, however Milley said there was a "strong possibility" there may have been a "verbal altercation" that preceded the shooting.

The general also said there is, "no indication he was targeting specific people."

"We have very strong evidence he had a medical history ... an unstable psychiatric condition" and said he believed that was the "fundamental underlying cause" of his lethal outburst.

Lopez, 34, was, "undergoing a variety of treatments and diagnoses of mental health issues ranging from depression to anxiety to sleep disturbance," Army Secretary John McHugh said Thursday.

He had not been diagnosed with post traumatic stress disorder but was undergoing a diagnostic process to determine if he had it, officials said.

According to authorities, Lopez brought onto the base a semi-automatic, .45 caliber Smith & Wesson handgun he recently purchased but not registered with the Army.

Sources familiar with the investigation tell ABC News the gun used in Wednesday's Ft. Hood shooting was purchased at Guns Galore in Killeen, Texas. This is the same store where Major Nidal Hasan purchased the gun he used in 2009 massacre on the base.

Around 4 p.m., Lopez walked into a command building and opened fire, then left, got in his car and began driving while still firing shots. He got out of his car and walked into another building, still shooting.

Three people are still in critical condition, with injuries to the neck, spine, and abdomen, respectively, while the others are stable, hospital officials said Thursday.

The shooting stopped only when he was confronted by the MP.

"It was clearly heroic, what she did at that moment in time," Milley said.

McHugh said Thursday that Lopez was a native of Puerto Rico, where had served in the National Guard for nine years before enlisting in the Army in 2008. He had served in Sinai with the National Guard and then as a truck driver when he was deployed to Iraq, where he replaced another soldier and finished out that soldier's deployment for four months.

Lopez is married and lived with his wife, also a native of Puerto Rico, and young daughter off base. He relocated to Fort Hood in February and was previously stationed at Fort Bliss in Texas, Milley said.

Xanderia Morris, who lives near the Lopez family in Killeen, said Lopez's wife walked outside, looking distraught as news of the shooting spread.

"She told me that she hasn't talked to [her husband] since 3 o'clock and was hysterical," Morris said. "She was shaking and crying."

The shooting details were especially surprising for Morris because, she said, Lopez seemed normal, a devoted husband and father.

"He was always smiling and waving whenever I saw him, so I didn't think anything strange or unusual," she said.

A senior military official told ABC News that Lopez reported to the military last summer that he was suffering from a traumatic brain injury and was having trouble sleeping.

He had been seeing a psychiatrist as recently as last month, McHugh said, but Lopez had not shown any signs of likely violence toward himself or others. He had not been suicidal. The psychiatrists had planned to continue monitoring and treating him. The only medication that officials said publicly that he was prescribed was the sleep aid Ambien.

Wednesday's violence brought back sharp memories of November 2009, when Army psychiatrist Major Nidal Hasan opened fire inside Fort Hood, killing 13 people. It was later revealed it was known he had been in email contact with wanted terrorist Anwar al-Awlaki, and tried to contact other al-Qeada-linked figures.

Hasan was convicted and sentenced to death in August.

Gen. Ray Odierno said Thursday that the procedures put in place after the 2009 shoot, "did help us yesterday," and cited the alert procedures, the response and the training of response forces as preventing Wednesday's shooting from, "something that have could have been much worse."